The present invention is directed toward a wavelength division multiplexed (WDM) ring system.
Optical communication systems are a substantial and fast growing constituent of communication networks. The expression "optical communication system," as used herein, relates to any system which uses optical signals to convey information across an optical waveguiding medium, for example, an optical fiber. Such optical systems include but are not limited to telecommunication systems, cable television systems, and local area networks (LANs).
Currently, many optical communication systems are configured to carry an optical channel of a single wavelength over one or more optical waveguides such as optical fibers. To convey information from multiple sources, time-division multiplexing (TDM) is frequently employed. In TDM, a particular time slot is assigned to each signal source with the complete signal constructed from portions of the signal collected from each time slot. While this is a useful technique for carrying plural information sources on a single channel, it is prohibitively expensive to continue to increase the TDM data rate.
While the need for communication services increases, the current capacity of existing waveguiding media is limited. Although capacity may be expanded (e.g., by laying more fiber optic cables), the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of existing optical waveguides.
Wavelength division multiplexing (WDM) is being explored as an approach for economically increasing the capacity of existing fiber optic networks. WDM systems typically include a plurality of transmitters, each respectively transmitting signals on a designated wavelength. As a result, fiber capacity can be increased by a multiple equal to the number of channels.
WDM systems have been deployed in long distance networks in a point-to-point configuration consisting of end terminals spaced from each other by one or more segments of optical fiber. In metropolitan areas, however, WDM systems having a ring or loop configuration are currently being developed. Such systems typically include a plurality of nodes located along the ring. At least one optical add/drop element, associated with each node, is typically connected to the ring with optical connectors. The optical add/drop element permits both addition and extraction of channels to and from the ring. One of the nodes, referred to as a hub or central office node, typically has a plurality of associated add/drop elements for transmitting and receiving a corresponding plurality of channels to/from other nodes along the ring.
Each channel in a WDM system is typically transmitted at a unique wavelength. Conventional filters, such as dielectric filters, can be provided within the add/drop elements in order to facilitate adding and/or dropping of individual channels, while allowing the remaining channels to continue along the ring. Each filter and its associated optical connectors, however, imposes an incremental loss on optical signals propagating along the WDM ring. Accordingly, a particular channel traversing a given number of filters along the ring can incur significantly more loss than other channels traversing fewer filters. As a result, the "worst-case" loss for one channel passing through a maximum number of filters can be so high as to limit the ring circumference.